83

Gold Deposits of the Flin Flon - Amisk Lake Area by J.G. Pearson and A.G. Galley1

Pearson, J.G. and Galley, A.G. (1985): Gold deposits of the Flin Flon - Amisk Lake area: in Summary of Investigations 1985, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Reper! 85-4.

The purpose of this report is to examine the evolution of gold deposits in the Flin Flon - Amisk Lake area.

General Geology The Flin Flon belt comprises an assemblage of Aphebian submarine volcanic and associated sedimentary rocks, the Amisk Group, and an overlying group of immature, coarse. terrestrial sediments, the Missi Group (Fig. 1). The belt is in gradational contact to the north with the Kisseynew metasedimentary terrain and to the south is unconformably overlain by Ordovician limestone. It extends from the Wekusko Lake area of Manitoba to the Sturgeon Weir River in Saskatchewan.

Stauffer and Mukerjee (1971) recognized three main phases of Hudsonian deformation within the Missi Group. In addition. two generations of faulting have been recorded (Byers and Dahlstrom, 1954; Byers et al.. 1965): an early generation coeval with the second phase deformation (P2) and a later generation associated with the third deformation (P3).

Four ages of igneous bodies intrude the supracrustals (Stauffer, 1974; MacQuarrie, pers. comm.). The oldest intrusions are subvolcanic granitic plutons (Cliff Lake pluton. Boot Lake pluton?). Gabbros followed by two ages of granites, the latest of which includes the Reynard , Annabel, Neagle and Phantom Lake plutons, were emplaced subsequently. Recent studies indicate that the Boot Lake pluton may also be a late intrusion.

Amisk volcanism occurred at 1.88 to 1.89 Ga with the last intrusive events occurring at 1.82 Ga (MacQuarrie, unpubl. data; Stauffer, 1985).

Douglas Lake - Phantom Lake Area Stratigraphy

The stratigraphy throughout this area can be broadly correlated. In general, the base of the sequence comprises massive and pillowed basalts and associated fragmental rocks. These are overlain by rhyolite flows and pyroclastic rocks. graphitic argillite and a thick sequence of bedded, waterlain luffs (Fig. 2).

1Economic Geology and Mineralogy Division. Geological Survey of Canada. Project funded under the Saskatchewan component of the Canada-Saskatchewan Subsidiary Agreement on Mineral Development 1984-89.

The volcanic rocks are intruded by several bodies of gabbro and diorite, the Boot Lake and Reynard Lake granite plutons and the Phantom Lake porphyritic granite.

Gold Mineralization

There are two generations of gold mineralization respectively predating and postdating emplacement of the Phantom Lake pluton.

The earlier generation is manifested by occurrences (Newcor mine, Car, Unity, Bomber Lake) in the lower basalt unit east of Douglas Lake (Fig. 2). Narrow (1 m wide) quartz-sulphide veins occupy northwesterly striking fractures. Within the veins the sulphides form massive zones (Newcor), anastomosing stockworks (Cor. Bomber Lake) and vug fillings (Unity, Bomber Lake). In places they show a zonation. Where veins crosscut mafic volcanics. wall rocks are altered to fissile chloritic schist or chlorite-plagioclase-epidote schist; where quartz-porphyritic rhyolite is crosscut, as at the Unity occurrence, wall rock alteration is marked by development of chlorite and sulphides on the foliation planes.

The gold mineralization that postdates the Phantom Lake granite is of three main types: 1) volcanic-hosted carbonate-quartz-su lphide zones (Rio, Macmillan, Dee, Wekach Lake). 2) fracture-controlled quartz-sulphide veins in the Boot Lake' pluton (Phantom Lake, Henning-Maloney, Yellow Jacket) and 3) fracture-controlled disseminated sulph ide zones in the Phantom Lake granite (IMC).

The Rio deposit (Pearson, 1984; Middleton, 1985), which is representative of the volcanic-hosted deposits, lies on the Rio Fault (Fig. 3) . T he hanging wall of the fault comprises basaltic lavas and fragmental rocks. whereas the Boot Lake pluton and included basaltic rocks occupy the footwall or southeast side of the fault. Northeast of the deposit there is little alteration along the fault and shear planes are coated with chlorite and ankerite, whereas to the southwest the fault is marked by a zone of intense ankeritization, silicification and quartz veining. The Rio deposit occurs w ith in a 90 m wide zone of carbonate--quartz alteration elongated along the trace of the fault, especially where included basalt is in contact with similar rocks in the hanging wall (Fig. 3). Within this alteration envelope there is no distinct fault plane, but rather a wide zone of fracturing.

The hanging wall orebody is a poorly defined zone of pyrite-gold mineralization grading 2 to 4 g/ t Au in which gold values vary sympathetically w ith pyrite

84

LAKE

!. j 41.11'

/1 l(

) /1,)' ·, ,_)

LEGEND

Amisk Group- Epiclastics

2 Amisk Group- Felsic Volcan ics

Am1sk Group- Ma fie Volcanics

• Gold O c: curr11nc11

LJ A,ea , Mopped ,n De toil

On~ov ician Dolom1fe

6 Falsie Intrusives

5 Mafic lnfrusf..,es

4 MiHi Grovp- C onglomerote and Greywa cke

Figure 1- Geology and go ld occurrences in the Flin F/on-Amisk Lake Area. 1, Newcor Mine; 2, Hennrng-Maloney Mine; 3, Rio Deposit; 4, Unity; 5. Cor; 6, Bomber Lake; 7. Wekach Lake; 8, Ange/ski; 9. Phantom Lake Mine; 10, MacMillan Mine; 11, North Phantom Lake; 12, Yellow Jacket; 13, Phantom-McRobbie Lake; 14, Dee; 15. IMC; 16, Dodo; 17a, Mosher Lake East; 17b, Mosher Lake North; 17c, Mosher Lake South; 18, Mitchell; 19, Sye; 20, Amisk Syndicate: 21, VM. Group; 22, Derby (Hoodoo); 23, Lucky Strike; 24, Royal; 25, Kent; 26, Duplex; 27, Monarch Mine; 28, Amisk Gold Syndicate; 29, T.C.A. ; 30. Laurel; 31 , Beaver; 32, Hannay Island; 33. Waverly Island: 34, Wmg; 35, Bud; 36, Graham Mine; 37. Golden Cross; 38, Robinson Creek; 39, Ace; 40, Sonora.

concentration. Mineralization is commonly concentrated along zones or fractures trending at 1700. The lower or footwall orebody is of considerably higher grade (10 to 20 git Au) and comprises narrow (30 to 50 cm wide), massive, pyrite and quartz-pyrite veins also striking at 1700.

Type 2 gold mineralization is represented primarily by discrete gold-quartz-copper-tungsten-molybdenite veins in the Boot Lake pluton (Galley and Franklin, this volume). At the Phantom Lake mine it is hosted by a medium-grained hornblende granodiorite (Fig. 2). The granodiorite and adjacent porphyritic Phantom Lake granite are crosscut by two planar fracture sets, the older of which strikes west-northwest and north-northwest and the younger east-northeast and north-northeast. The

veins are controlled by the older set of fractures, and do not appear to cross the contact with the Phantom Lake granite. Pyrite with minor chalcopyrite, arsenopyrite, sphalerite and molybdenite and accessory feldspar and purple fluorite occur in the veins.

Type 2 mineralization is also found as a vein stockwork in a 10 m wide zone striking parallel to the west-northwest fracture set. There appear to be at least three vein types striking roughly at right angles to the strike of the zone within the stockwork. Quartz-feldspar veins are crosscut by thinner quartz-chalcopyrite-pyrite veins which are in turn cut by even thinner quartz-chlorite-ve,ns. Lower temperature alteration assemblages, consisting of epidote, pyrite. quartz, hematite and white clay, and controlled by

LEGEND ~ B oundary Intrusion s

W Phantom L . Granite [I]Grano

~Gabbro

Amisk Group Q_] Tuf!

IT] R hyolile

Q]oacl te

[I] Frog me nt ol Rocks QJ Basalt

,,/ Fault

2 k m

7

7

7

7

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7

7

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85

A

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-~~-,.-

60

Figure 3 - Geological sketch of the area around the Rio deposit.

The lower part of the 'Amisk Volcano' comprises several kilometres of submarine basalts and associated breccias and fragmental rocks (Byers et al., 1956; Longeriau, 1980; Ayres, 1977; Ayres et al., 1981; MacDougall et al., 1979; Parslow and Gaskarth, 1984; Fox, 1976; Walker and Watters, 1982; van Wagoner and van Wagoner, 1981). It is overlain by abundant luffs mixed with subaerial and subaqueous flows.

These rocks are folded by second deformation (P2) structures with northerly-trending axial planes and disrupted by coeval north- to northwesterly-trending faults (Mosher Lake, MacDonald Creek, West Channel), typified by several parallel and tangential branches.

Gold Mineralization

Two types of gold deposits are present: 1) gold-quartz-carbonate-sulphide veins (Mosher Lake, Mitchell, Sye, Robinson Creek, Graham, Golden Cross, Duplex, Kent, Monarch Mine, Waverly, Sonora) and 2) disseminated gold-sulphide zones (Laurel Lake, Gull Island, Beaver and Amisk Gold Syndicate) (Pearson, 1980, 1983). Type 1 deposits, the gold-quartz veins, are invariably located along the flanks of major faults, as at the Monarch Mine, Sonora. and Robinson Creek, or within minor subsidiary faults, as at the Graham mine, Duplex and Golden Cross. Deposits occur variously within Amisk Group and Missi Group rocks; however, despite a wide variety of host rocks, the associated alteration is remarkably consistent and typified by development of pervasive iron carbonate (ankerite) and fine-grained silica-sericite-tourmaline. In the most intensely altered zones (Sye, Mosher Lake South, Robinson Creek-Bleiler zone (Pearson, 1982, 1983)), the host basalts are transformed to a very fine

86

grained, fissile, grey, carbonate-chert rock and fractures are filled by quartz and quartz-carbonate veins.

The two deposits in the Missi Group (Robinson Creek, Graham) are typical gold-quartz-carbonate vein systems developed in proximity to major faults (Pearson, 1982, 1983). Veins are folded by second phase structures suggesting that the main period of gold-quartz vein formation was coeval with or postdated faulting, but occurred prior to the major second-phase folding.

Type 2 deposits occur in the West Channel Area of Amisk Lake. Around Laurel Lake and Gull Island (Fig. 4) the lowest part of the stratigraphic sequence is exposed as massive, pillowed and variolitic basalts. These are intruded by massive quartz porphyry which in the area west of Laurel Lake is flanked by crystal and lapilfi tufts grading into fine ash tuft. The felsic tufts are in turn overlain by a thick sequence of fine-grained, bedded to laminated mafic tufts. These rocks are folded by northerly-plunging upright P2 folds with steeply dipping axial surfaces. and cut by a series of north- and northwesterly-trending fractures which are subparallel and tangential to the West Channel Fault system.

Within this setting there are a number of gold occurrences showing a variety of different accessory minerals. Thin veins of massive calcite-quartz-gold±pyrite±pyrrhotite±chalcopyrite±sphalerite±galena are present in fractured basalt, whereas in quartz and quartz-feldspar porphyry there are zones of disseminated fine-grained pyrite. gold mineralization. Associated with this fracture-controlled mineralization is an intense iron carbonate and sericite alteration characterized by disseminated magnetite (partially altered to hematite) and chforite-filfed fractures. In massive quartz porphyry, vuggy fractures are lined with pyrite, tetrahedrite-tennantite, enargite and rarefy gold. They are flanked by successive zones of fine-grained chlorite alteration and sericite alteration. Lapilli tufts contain stratabound disseminated pyrite-gold mineralization, in which small lenses of sphalerite and green mica are developed. The mineralization is enveloped by a pervasive sericite and iron carbonate alteration and folded by second-phase structures ( P2).

Discussion

In the Douglas-Phantom Lakes area, the early generation of quartz-sulphide vein deposits was emplaced during the formation of second-phase northwesterly-trending fractures, prior to intrusion of the Phantom Lake granite. Veins lie within narrow alteration zones, have similar mineralogy and are commonly vuggy, indicating that they were introduced at rather shallow depths (Boyle, 1979). Brecciated veins in the Unity and Bomber Lake occurrences and the boudinaged nature of the Car occurrence suggest intermittently active fractures.

The later generation of deposits is more complex.

87

i::?s?J??:::}:~::::. \· ............ . GEOLOGY OF LAUREL LAKE-\.::: :: : : :: :: :: : :::,;;;~:::: ~~-~~:-, ~ • 1 • • • • • (, I -.I -..-"'::"" :, I ' \~\ .,\ ; G U LL I SL AND AREA

LEGEND

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1/1 i/ ,1 ~\ :/:?>>> t )ifI{f SCALE I e./ / ) ' ' .. ' . ' .. ' . I ·>,-::_~!°-·;! I~ / · ::::::::::::::::::-. L AK EI ,'-';'·>~'-·,•,

/ - .. _j /1 )~/ ..:::::::::::::::::::: f -~!{tit 100 100 200 m 0

D I '.'·-:.f. 1. ·• · ....... · ... · ....... ·-:.' 7 / ,,).~·,:2-; .· ,I tf{t\{\\\\\;\\\\\\\\\f) /' Ji!~;

7

Figure 4 - Geology of the Laurel Lake area in the West Channel - M,ssi Island area of Amisk Lake.

Deposits show differing ore mineralogy, gangue mineralogy, host rock and alteration assemblages, but are consistent in postdating the Phantom Lake granite and apparently in containing tungsten. They are considered as a product of the same porphyry system.

Observations made at and near the Phantom Lake mine and the IMC deposit suggest progressive alteration, from high to low temperature. along successive generations of fractures. The older fractures trend west-northwest and north-northwest, and contain quartz, pyrite. chalcopyrite, arsenopyrite, molybdenite, feldspar and fluorite, as well as chlorite and scheelite. This type of mineralization does not appear to cross the contact of the Phantom Lake pluton. The later east-northeast and north-northeast fractures, containing epidote, pyrite, quartz, hematite and clay minerals, however, occur in both the Boot Lake and Phantom Lake plutons and indicate overprinting of potassic and phyllic alteration by propylitic alteration.

The Amisk Lake gold deposits all occur in extrusive volcanic rocks, subvolcanic intrusives and volcanogenic sediments. These rocks were variably fractured

subsequent to, or contemporaneous with, the formation of the quartz porphyry (rhyolite) dome (Fig. 5) at Laurel Lake. Hydrothermal fluids followed the fracture conduits provided within the brittle dacite and rhyolite porphyries, altering and mineralizing them. They also percolated into the more porous beds within the flanking rhyolitic tuft horizons. Basalts underlying the felsic dome contain some base metal rich veins, the genesis of which may reflect a differing response to the same hydrothermal regime.

The fracture-cont rolled gold-quartz veins representing the other type of gold mineralization in the Amisk Lake area are also thought to be products of this primary hydrothermal system. Fluids followed the major fracture zones, which formed conduits or channelways and gave rise to the gold-quartz mineralization in response to falling pressure and/or temperature. These deposits must have formed in response to early movements along the major faults prior to the second-phase deformation. This infers that the hydrothermal system at Laurel Lake was active for an extended period of time prior to Hudsonian orogeny.

88

LJ Mafic Tuff [}{:~ Rhyolite - ash tuft

~ - lopi II i tuff

Q -massive GJ Basalt

Alteration

Figure 5 - Schematic diagram illustrating the setting and controls of alteration and mineralization m the Laurel Lake area. Note that the alteration patterns follow fractures and porous beds within the fragmental apron flanking the quartz porphyritic rhyolite dome.

Acknowledgements

Although much of this material has been presented previously, new observations and ideas, particularly from the Phantom Lake mine and the Laurel Lake deposit, by A.G. Galley of the GSC and F.H. MacDougall of SMDC respectively, have lead to new interpretations and a refinement of the previously presented hypothesis. A more complete description of this information will be included in a paper entitled "The Geology and Evolution of Gold Deposits in the Flin Flon - Amisk Lake Area" by J.G. Pearson, F.H. MacDougall and A.G. Galley to be included in the CIM volume "Gold in the Western Shield".

References

Ayres, L.D. (1977): A transition from subaqueous to subaerial eruptive environments in the middle Precambrian Am1sk Group at Amisk Lake, Saskatchewan - a progress report; lnstit. Precambrian Stud., Annu. Rep., p36-51.

Ayres, L.D., van Wagoner, N. and van Wagoner, S. (1981 ): Physical volcanology of the Amisk Lake volcano; in Summary of Investigations 1981, Sask. Geol. Surv., Misc. Rep:81-4, p47-51.

Baldwin DA (1980): Porphyritic intrusions and related mineralization in the Flin Flan volcanic belt; Manit. Miner. Resour. Div., Econ. Geol. Rep. ER 79-4.

Boyle, R.W. (1979): The geochemistry of gold and its deposits; Geol. Surv. Can., Bull. 280, 584p.

Byers, A.R. and Dahlstrom, C.D.A. (1954): Geology and mineral deposits of the Amisk Lake - Wildnest Lake area, Saskatchewan; Sask. Dep. Miner. Resour .. Rep. 14, 177p.

Byers, A.R., Kirkland, S.J.T. and Pearson, W.J. (1965): Geology and mineral deposits of the Flin Flan area, Saskatchewan; Sask. Dep. Miner. Resour .. Rep. 62, 95p.

Fox, J.S. (1976): Some comments on the volcanic stratigraphy and economic potential of the west Amisk Lake area, Saskatchewan; Sask. Res. Counc., Gire. 9.

Longeriau, S.J. (1980): Structure and metamorphism of the northeast Amisk Lake area; unpubl. M.Sc. thesis, Univ. Sask., 119p.

MacDougall, F.H., Parslow, G.R. and Watters. B.R. (1979): Flin Flon base metals project; in Summaiy of Investigations 1979, Sask. Geol. Surv., Misc. Rep. 79--10, p116-119.

Middleton, TA (1985): The relationship between the structural setting and the development of the alteration zone at the Rio Gold Mine, Saskatchewan; unpubl. B.Sc. thesis, Univ. Waterloo, 54p.

Parslow, G.R. and Gaskarth, J.W. (1984): Geochemistry of the East Amisk area; Sask. Energy Mines, Open File Rep. 84-23; 93p.

Pearson, J.G. (1979): Flin Flon gold prospects; in Summary of Investigations 1979, Sask. Geol. Surv., Misc. Rep. 79-10, p145-151.

_ ____ (1980) : Flin Flon gold project; in Summary of Investigations 1980. Sask. Geol. Surv .. Misc. Rep. 80-4, p70-80.

_ _ _ _ _ (1981 ): Gold metallogenic studies: Phantom Lake-Schist Lake area; in Summary of Investigations 1981 , Sask. Geol. Surv., Misc. Rep. 81-4, p97-100.

_ _ _ _ _ (1982): Gold metallogenic studies: Amisk Lake east area; in Summary of Investigations 1982, Sask. Geol. Surv .. Misc. Rep. 82-4, p64-70.

_ ____ (1983) : Gold metallogenic studies: Flin Flon - Amisk Lake area; in Summary of Investigations 1983, Sask. Geol. Surv .. Misc. Rep. 83-4, p67-74.

_ _ _ __ (1984): Gold metallogenic studies: the Rio deposit; in Summary of Investigations 1984, Sask. Geol. Surv., Misc. Rep. 84-4, p123-126.

Stauffer, M.A. (1974): Geology of the Flin Flon area: a new look at the Sunless City; Geosci. Can., v1 , p30-35.

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_ _ _ __ (1985): Manikewan: an early Proterozoic ocean in central Canada, its igneous history and orogernc closure; Precambrian Res., v25, p257-281 .

Stauffer, M.A. and Mukerjee, A.C. (1971) : Superimposed deformations in the Missi metasedimentary rocks near Flin Flon, Manitoba; Can. J. Earth Sci., v8, p217-242.

Stewart, J.W. (1977): MEA-1 Gold evaluation program; ;n Report of Activities 1977; Manit. Miner. Resour. Div., p105-108.

van Wagoner. N. and van Wagoner. S. (1981): Shallow water volcanism and sedimentation in a portion of the Flin Flon-Snow Lake greenstone belt; lnstit. Precambrian Stud .. Annu. Rep., p71-88.

Walker, D. and Watters, B.R. (1982): Geochemistry of metavolcanic rocks, Amisk Lake west area; in Summary of Investigations 1982. Sask. Geol. Surv., Misc. Rep. 82-4, p24-30.

Wallster, D.E. (1979): Geology reviewed for the Dion Lake region with specific reference to a possible Aphebian porphyry-type copper deposit; unpubl. B.Sc. thesis; Univ. Western Ontario, p141.